1. Technical Field
This invention pertains to the technical field of documents designed to store information in a manner not visible to the unaided human eye, for the purpose of preserving the confidentiality of the information, and limiting its use to authorized persons. Specifically, the invention pertains to a label or identification card bearing concealed information in the form of a bar code to control access to an area or a machine, for example. Such a label or card may be used in a factory, an office, a military installation, or any other place in which security is desirable.
2. Background Art
Various card systems are known in the prior art, by which optically readable information may be stored in a concealed manner. Typically, an identification card may contain indicia which are rendered perceptible only when exposed to light beyond the visible spectrum. The indicia may be covered by a barrier permeable to invisible light, such as ultraviolet or infrared, while being opaque to light in the visible spectrum.
The indicia may be, for example, holes in a metal plate, fluorescent material, spots of ink, or a plastic with a sharp single-frequency absorption characteristic. See, e.g., U.S. patents:
______________________________________ 3,239,815 3,477,156 3,802,101 3,245,697 3,536,894 3,829,662 3,455,577 3,621,249 3,919,447 3,468,046 3,640,009 4,044,231 ______________________________________
The systems known in the prior art are subject to various disadvantages. A barrier sufficiently permeable to invisible light, while at the same time opaque to visible light, is generally costly. If the opaque barrier is not sufficiently permeable to invisible light, then the reader which scans the invisible light must be built to much more demanding specifications at commensurately higher cost, or its performance will be detrimentally affected by the less than adequate permeability.
Those systems which require the use of fluorescent material to render the indicia perceptible are disadvantageous, in that such a material may be costly, especially if it must be incorporated into each card or label, of which there may be hundreds or even thousands in a single installation.
Those systems which employ holes in a metal or plastic plate or sheet as the indicia are disadvantageous, in that such holes can not easily be closely-spaced. The closer the spacing, the more difficult it is to maintain manufacturing tolerances. Accordingly, the information density is limited. Such systems are particularly unsuitable for use with bar-coded information.
It is also known in the art to use ID cards having a magnetic stripe on which is recorded data for reading by a magnetic head. Such data is not visible to the unaided eye. However, the magnetic strip is subject to attack by acetone and similar chemicals, which take off the layer with the magnetic oxide and hence destroy the recording. An overlay of plastic can protect against acetone but it cannot be used because of the absolute necessity for close spacing between the magnetic read head and the magnetic stripe. With the ordinary read heads employed, lack of close spacing degrades the signal too much, and the only alternative is much more expensive read heads which could accurately read across the gap.
A magnetic reader for this type of card is generally in the form of a slot into which the card is inserted. A spring loaded plate forces the card into an arc, hence its need for flexibility. The card is in essence wrapped around the read head to assure adequate and continuous firm contact between the head and the magnetic stripe. The sides of the slot align the card with respect to the head to prevent skew errors. The closeness between the read head and the card means that there can be no environmental barrier between the slot and the head, so the latter is subject to dirt, moisture and other environmental degradation. The necessary thinness of the card leads to a relatively short lifetime. Perhaps most importantly, the magnetic data is subject to degaussing by any strong magnetic field, including those normally encountered in plants due to machinery.